The predominant source of methane, an abundant low molecular weight alkane, is natural gas, which is found in porous reservoirs generally associated with crude oil reserves. From this source comes most of the natural gas used for heating purposes. Quantities of natural gas are also known to be present in coal deposits and are by-products of crude oil refinery processes and bacterial decomposition of organic matter. Natural gas obtained from these sources is generally utilized as a fuel at the site.
Prior to commercial use, natural gas must be processed to remove water vapor, condensible hydrocarbons and inert or poisonous constituents. Condensible hydrocarbons are generally removed by cooling natural gas to a low temperature and then washing the natural gas with a cold hydrocarbon liquid to absorb the condensible hydrocarbons, which comprise ethane and heavier hydrocarbons. Natural gas can be treated at the wellhead or at a central processing station. The processed natural gas comprises predominantly methane, and minor quantities of ethane, propane, butane, pentane, carbon dioxide and nitrogen. Generally, processed natural gas comprises from about 50 volume percent to more than 95 volume percent methane.
Natural gas is used principally as a source of heat in residential, commercial and industrial service. Methane also has commercial uses in the chemical processing industry. The largest use for methane, other than as a primary fuel, is in the production of ammonia and methanol. Ammonia is a basic ingredient of fertilizers and is also a common feedstock in the production of petrochemicals, such as acrylonitrile and nylon-6. Methanol is a precursor material for products, such as formaldehyde, acetic acid and polyesters.
Methane has also been used as a feedstock for the production of acetylene by electric-arc or partial-oxidation processes. Another commercial use for methane is in the production of halogenated products, such as methyl chloride, methylene chloride, chloroform and carbon tetrachloride. Methane also reacts with ammonia to produce hydrogen cyanide.
Most processed natural gas is distributed primarily through an extensive pipeline distribution network. As gas reserves in close proximity to gas usage decrease, new sources in distant locations require additional transportation. Distant sources may not be amenable to transport by pipeline, such as sources that are located in areas requiring economically unfeasible pipeline networks or in areas requiring transport across large bodies of water. This concern has been addressed in several ways.
One solution has been to build a production facility at the site of the natural gas deposit to manufacture one specific product. This approach is limited because the natural gas can be used only for one product, preempting other feasible uses for methane.
Another approach has been to liquify methane and transport the liquid methane in specially designed tanker ships. Natural gas can be reduced to 1/600th of the volume occupied in the gaseous state by cryogenic processing, and with proper procedures, safely stored or transported. The processes for liquifying natural gas to a temperature of about -162.degree. C., transporting, and revaporizing are complex and energy intensive.
Still another approach has been to convert methane to higher order hydrocarbon products that can be easily handled and transported, preferably hydrocarbon products that exist in a liquid state. The conversion of methane to higher order hydrocarbons, especially ethane and ethylene, would retain the versatility of the material for use as a precursor in chemical processing. Known dehydrogenation and polymerization processes are available for further conversion of ethane and ethylene to liquid hydrocarbons, such as the processes taught by Chu in U.S. Pat. No. 4,120,910 and Chen et al in U.S. Pat. No. 4,100,218, both disclosures being incorporated herein by reference. In these ways, easily transportable commodities may be derived directly from natural gas at the wellhead. The drawback to implementing such a process, however, has been in obtaining a sufficient conversion rate of methane to higher order hydrocarbons.
Catalytic coupling of alkanes is known in the art. While it is possible to convert alkanes to higher order hydrocarbons, major undesirable by-products are CO and CO.sub.2.
The catalytic oxidative coupling of methane at atmospheric pressure and at temperatures of from about 500.degree. to 1,000.degree. C. has been investigated by several researchers. G. E. Keller and M. M. Bhasin reported the synthesis of ethylene via oxidative coupling of methane over a wide variety of metal oxides supported on an alpha-alumina structure, Journal of Catalysis 73, 9-19 (1982). This article discloses the use of single component oxide catalysts that exhibited methane conversion to higher order hydrocarbons at rates no greater than 4 percent. The process by which Keller and Bhasin oxidized methane was cyclic, varying the feed composition between methane, nitrogen and air (oxygen) to obtain higher selectivities.
West German Pat. No. DE 32 37 079.2 to Baerns and Hinsen reports the use of supported, single component oxide catalysts. The process taught by Baerns and Hinsen utilizes low oxygen partial pressure to give a high selectivity for the formation of ethane and ethylene. The conversion of methane to such desired products, however, remains low, on the order of from about four to about seven percent conversion.
U.S. Pat. Nos. 4,443,644-4,443,649 inclusive to Jones et al teach methods for synthesizing hydrocarbons from methane by contacting methane with a single component metal oxide at temperatures between about 500.degree. and 1,000.degree. C. The processes taught in these patents produce high selectivities to higher order hydrocarbons but at very low conversion rates, on the order of less than 4 percent overall conversion to higher order hydrocarbons. PbO is used as the single component metal oxide in U.S. Pat. No. 4,443,647.
U.S. Pat. No. 4,495,374 discloses the use of an alkaline earth metal or a component thereof with a reducible metal oxide to improve the conversion of methane to higher hydrocarbons. U.S. Pat. No. 4,499,322 discloses the use of an alkali metal or a compound thereof as a promoter to improve methane conversion. In each case conversion is carried out at a temperature in the range of about 500.degree. to 1000.degree. C.
Through experimentation with different types of catalysts under various reaction conditions, it was discovered that catalyst activity can be increased by carrying out methane coupling reactions at relatively high temperatures, that is, temperatures above about 900.degree. C. The formation of hydrocarbons is favored at the higher temperatures, while smaller amounts of CO and CO.sub.2 are formed as undesired by-products.
These results suggest that high yields of higher order hydrocarbons could be obtained if an alkane was upgraded in the presence of a PbO-containing catalyst at a high temperature, such as about 900.degree. to 950.degree. C. However, PbO (litharge) melts and volatilizes at a temperature of about 888.degree. C. Lead melts at an even lower temperature of 327.degree. C. Volatilization leads to a loss of catalyst from the reactor and a resulting decrease in reactor productivity. More importantly, release of Pb and PbO from the reactor into the environment cannot be tolerated. For these reasons, reaction temperatures above about 850.degree. C. can only be employed for reaction times that are sufficiently short to avoid volatilization of PbO. Such short reaction times are unsuitable in a commercial operation.
It has been suggested that a PbO-containing catalyst may be employed in a commercial alkane coupling process at a temperature somewhat below the volatilization temperature of the PbO, such as about 800.degree. to 850.degree. C., but the use of lead oxide even at these temperatures is somewhat limited. The alkane coupling reaction is an exothermic reaction accompanied by the release of large quantities of heat. It is very difficult to control the temperature throughout the reactor within a narrow temperature range. For this reason it is not unusual for the reactor to have a temperature profile that includes high temperature zones or hot spots at or above 888.degree. C. The PbO can thus volatilize in these zones. This problem exists whether the lead oxide catalyst is supported or unsupported. Thus, the use of the known PbO-containing catalysts in high temperature, commercial alkane coupling processes is limited.
There exists a need in the art for a catalyst suitable for upgrading alkanes to higher order hydrocarbons at high reaction temperatures. The catalyst should exhibit high temperature stability and high activity in converting alkanes to higher order hydrocarbons, including liquid hydrocarbon products. The catalyst should retain its chemical activity, structural integrity and mechanical properties at temperatures up to about 1550.degree. C. In addition, volatilization of Pb and PbO from the catalyst should be negligible at temperatures up to about 1550.degree. C. The catalyst should have long catalyst life and should be suitable for use in natural gas conversion to higher order hydrocarbon products at high selectivities and at commercially feasible methane conversion rates. There also exists a need in the art for a process to upgrade methane to produce predominantly ethane and ethylene.